The invention relates to a process in which thermally unstable solvent mixtures, whose components boil in closely related boiling ranges, are continuously recovered from higher boiling hydrocarbon mixtures by evaporating the solvents in several evaporators connected in series under increasing pressure and temperature. The evaporated solvent components are withdrawn as vapor from the upper part of the respective evaporator, while the bottoms product accumulates at the base of the evaporator.
Oil-solvent mixtures are often processed to separate the oil from the solvent. Such mixtures occur, for example, in the solvent dewaxing of mineral oil. One such process, known as the Di-Me solvent dewaxing process, utilizes a solvent mixture of dichloromethane and 1, 2 dichloroethane (for simplicity and in accordance with common usage, from which the process obtains its name, these components will be referred to hereinafter as "methylene chloride" and "dichloroethane", respectively). One embodiment of the Di-Me process is discussed in 42 "Hydrocarbon Processing & Petroleum Refiner", No. 12, 104-106 (1963). This process may be used to dewax low and high viscosity lubricating oils or to deoil paraffin waxes, slack waxes and similar oily wax mixtures. The raw distillate is diluted with the solvent mixture and then chilled. The function of each component in the solvent mixture is different. Dichloroethane is a wax precipitate while methylene chloride is a solvent for the oil. The wax, oil and solvent mixture is passed to rotary filters where it is separated into wax (containing solvent) and a filtrate mixture of wax-free oil and solvent. The wax cake is washed with cold solvent. The filter wash obtained forms part of the solvent added to the raw distillate. The filtrate and wax cake are processed in separate but similar solvent recovery systems where a series of evaporators separates the solvent from the wax and from the oil. Final traces of the solvent are removed by steam stripping.
In the recovery of oil-solvent mixtures, which occur, for example, in the solvent dewaxing of mineral oils and contain lower boiling chlorinated hydrocarbons, such as methylene chloride and dichloroethane, the solvent components are evaporated off in series connected evaporators with several pressure stages. Thus, from DT-PS (German Federal Republic patent) No. 871,742 it is known to connect in series with a first evaporator, a second evaporator operating at a higher pressure, and utilizes its vapor as a heating means for the inflow to the first evaporator. The bottoms product from the first evaporator is heated with a separate heating agent, such as, for example, steam, before it is introduced into the second evaporator. The evaporation temperature in the second evaporator is adjusted via the pressure, and is governed by the evaporation temperature of the solvent components and the desired amount of solvent which is to be removed in the first evaporator. For example, the second evaporator is operated at a temperature of about 130.degree. C. and at a pressure of about 4 atmospheres. The inlet temperature of about 132.degree. C. required in the second evaporator in order to fully evaporate methylene chloride and dichloroethane is obtained by heating the solution in a heat exchanger connected directly to the second evaporator, and which is charged with steam at a temperature of 145.degree. C. The tube wall temperatures in the heat exchanger obtained with this steam temperature accelerate the decomposition of the thermally unstable methylene chloride. This results in solvent losses and corrosion damage to the apparatus and piping, since highly reactive and corrosive hydrogen chloride is formed in the decomposition of methylene chloride.
The tendency of chlorinated hydrocarbons to decompose has been recognized for a fairly long time and is the subject of various modification proposals. According to a process disclosed in DT-PS No. 918,651, the danger of decomposition is said to be avoided if the first solvent recovery evaporator is operated at temperatures below 100.degree. C., preferably at 80.degree.-90.degree. C. At these temperatures a substantial part of the solvent, mainly the lower boiling, thermally unstable component, and a part of the water contained in the mixture, is driven off. However, it has been found that even this method does not completely avoid the tendency of chlorinated hydrocarbons to decompose. The bottoms product from the first evaporator still contains quantities of methylene chloride and has a volume which requires a corresponding large heating unit in order to heat the bottoms product to the desired inlet temperature of about 132.degree. C., to the second evaporator. The residence time required for this and the tube wall temperatures of about 145.degree. C. which are reached lead to an increased tendency of the methylene chloride still present in the solution to decompose. The residence time and the temperature determine the degree of decomposition of a thermally unstable component.
The object of the invention is to evaporate thermally unstable solvent components from higher boiling hydrocarbon mixtures in an economical process not subject to interruptions and disturbances. In particular, the object is to reduce the temperature and residence time in the solvent recovery from solvent dewaxing products so that thermal decomposition of the solvents is substantially eliminated and corrosion damage is avoided.